This invention relates generally to the field of Magnetic Resonance Imaging (MRI) More particularly, to time encoded Magnetic Resonance Imaging.
Magnetic Resonance Imaging (MRI) is known. Two-dimensional MRI (for example, for a 256.times.256 pixel image) generally comprises disposing a sample to be imaged in a static magnetic field oriented in a first direction, whereby the magnetization vector created by the atomic particles of the sample possessing a nuclear magnetic moment is constant in size and direction. The sample is then exposed to a first linear magnetic field gradient oriented in a second direction to define a slice. The sample is then excited with a radio frequency (RF) pulse, at the resonance frequency, the pulse being oriented in a third direction perpendicular to the first direction to rotate the magnetization vector 90.degree.. The sample is then exposed to a one-dimensional, uniform, gradient magnetic field oriented in a plane perpendicular to the first direction. A spin echo is produced by pulse or gradient reversal. At the same time the RF signals are detected, the sample is exposed to a second uniform magnetic field gradient oriented in a fourth direction perpendicular to the second direction. RF signals are emitted as a result of the spin echo, these RF signals are all received at the same time, however the atomic particles and the magnetization vector for each x-coordinate precesses with slightly different frequencies. Accordingly, each x-coordinate (in this example, 256 x-coordinates) has a different frequency. A Fourier transform is applied to this composite signal, whereby the signal for each x-coordinate is separated out from the composite signal.
The sample is again excited with a radio frequency (RF) pulse oriented in the third direction to rotate the magnetization vector 90.degree.. The atomic particles again jump to a higher energy state. The sample is then exposed to a one-dimensional, uniform, gradient magnetic field oriented in a plane perpendicular to the first direction. A spin echo is produced by pulse or gradient reversal. Before the RF signals are detected, the sample is exposed to a second uniform magnetic field gradient oriented in the fourth direction. RF signals are emitted as a result of the spin echo, these RF signals are all received at the same time, however the atomic particles and the magnetization vector for each y-coordinate precesses with slightly different phase shift. Accordingly, each y-coordinate (in this example, 256 y-coordinates) has a different phase shift. However, unlike the x-coordinates these phase shifts cannot be separated to identify each y-coordinate. These signals are defined by repeated exposure, as described above (in this example 256, one for each y-coordinate). In order to distinguish these phase shifts, the size or time duration of the exposure to the magnetic field in the y plane for each successive pulse sequence is varied. The above MRI process is more fully described in A Non-Mathematical Approach to Basic MRI, by H. Smith and F. Ranallo, Medical Physics Publishing Corp., 1989, the entirety of which is incorporated herein by reference
This process is very time consuming i.e., requiring 256 iterations to define the y-coordinates in a 256.times.256 image. This problem becomes clearly evident in the most common use of Magnetic Resonance Imaging (MRI), which is in the health care field. MRI has found wide-spread acceptance as a medical diagnostic technique for providing images of internal soft tissue structures of the human body. As is evident, in order to obtain these images a patient must be located within the MRI device for a considerable amount of time. Because of the requirement of a large static magnetic field, the patient is confined within a small space leading to a feeling of increased anxiety in some patients which is exacerbated by the lengthy duration of the imaging process. Additionally, this lengthy duration results in a high cost to the health care system because of, for example, operator time and the limited number of patients that can be examined in a given time. Thus making the MRI device less productive. In this day of health care reform due to sky rocketing health care costs, the impetus for this invention is most clearly evident.